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Green Infrastructure: The Effects of Urban Rail Transit on Air Quality


The transportation sector is a major source of air pollution worldwide, yet little is known about the effects of transportation infrastructure on air quality. In this paper we measure the effects of one major type of transportation infrastructure – urban rail transit – on air quality. Our approach uses the sharp discontinuity in transit utilization on the opening day of a completely new rail transit system in Taipei, Taiwan, to identify the air quality effects of rail transit infrastructure. Using hourly air quality data from Taiwan we have three central findings. First, we find that the opening of the Metro reduced air pollution from one key tailpipe pollutant, carbon monoxide, by 5 to 15 percent. Second, we find little evidence that the opening of the Metro affected ground level ozone pollution. Third, we find little evidence suggesting that automobile travelers adjusted their time or route of travel to the availability of rail transit. These findings shed new light on the determinants of air quality, and suggest that environmental impacts are important components of the social value of transportation infrastructure.